Add environment setup instructions for bootstrapping and environment variables

[jakepruitt]

For new contributors installing dependencies with ./bootstrap in mapnik core, there should be instructions for setting environment variables to point at the installed binaries. Specifically, there should be instructions to add the following to the contributor's ~/.profile file:

MASON_LINKED_ABS=/Users/jake/Projects/mapnik/mason_packages/.link
 export PATH=${MASON_LINKED_ABS}/bin:$PATH
 export PROJ_LIB=${MASON_LINKED_ABS}/share/proj/
 export ICU_DATA=${MASON_LINKED_ABS}/share/icu/54.1/
 export GDAL_DATA=${MASON_LINKED_ABS}/share/gdal

Alternatively, there could be Mac/OSX/Linux install instructions that explain exactly which packages to brew install or apt-get globally. cc/ @flippmoke @springmeyer

[springmeyer]

Thinking out loud: As far as the brew install I'm thinking that I'll provide a pull request to homebrew to make the homebrew package build against gdal, postgis, and cairo by default. Back when all homebrew installs were from source I think it made sense to reduce the default config. But now mapnik is packaged as a binary by homebrew, installs are fast and easy, and it would make sense for the default bottle to depend on gdal, postgis, and cairo. <-- this would make it so that all the tests in node-mapnik would pass against a homebrew default install rather than failing currently on the missing features.

[jakepruitt]

Hey @springmeyer, I was trying to run java recently, and it kept failing to find any java installations:

2016-01-16 14:57:49.959 java[10704:54199] [JVM Detection] FillMatcher: failed to create CFNumberFormatter
There do not appear to be any valid Java versions on this machine (NULL).

The problem was solved when I removed the ICU_DATA environment variable. I'm going to see if I can build and pass all of the tests without ICU_DATA set.