Removed fetching version from git repo

If  git isn't installed, OS X prompts the user to install Xcode.
We choose to remove this developer feature to make it better for the end user.

See #47
Close #178
Enhance documentation of parameterized Contexts.

-Point out that parameters can be Pint Quantity objects
-Clarify how to import custom context definitions
-add a new example 'chemistry' context with conversions for various
 concentration units
Fix error in chemistry context
Complete missing conversions in chemistry context.
Fix another error in chemistry context.
Define "Ω".
Define "ångström" and "Å".
Define prefix "µ".
fix another error in the chemistry context
Make "floz" the abbreviated version of "fluid_ounce" instead of the opposite

Fixes #187
Merge branch 'develop'
Merge branch 'doc_changes' of git://github.com/rsking84/pint into rsking84-doc_changes
Merge branch 'rsking84-doc_changes'