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.
fix another error in the chemistry context
Merge branch 'doc_changes' of git://github.com/rsking84/pint into rsking84-doc_changes
Merge branch 'rsking84-doc_changes'