Move MODEL_MAPPING to the config

In an effort to move table-level SQL details to the config so that the
only SQL details in the source code are predicate level.
Add specificity to boundarySql config string

So that another boundarySql field can be added to the config without
ambiguity.
Move select boundary statement to config
Test SQL creation logic for plots
Merge pull request #17 from RickMohr/test-plot-sql

Test SQL creation logic for plots
Merge pull request #15 from steventlamb/topic/more_sql_in_config

Topic/more sql in config
Add comments to clarify what we're doing in server.js

... also reorder config properties to help clarity
Merge pull request #18 from RickMohr/comment-tile-server

Add comments to clarify what we're doing in server.js