Simplify expression parsing

[asottile]

Resolves #218
Resolves #236

Also makes #81, #183, and #189 actually possible

High level changes:

• ${ foo } is a syntax error -- identifiers cannot start with whitespace
• $() and $[] are no longer valid placeholders, use ${} instead
• gettext parsing was removed -- this never worked entirely properly and you can do better by augmenting ._global_vars, compiling, and then scan the compiled output
• $foo()bar no longer compiles to foo().bar
• newlines in expressions are preserved moar (see Inline comment in multiline expression creates syntax error #236)
• escaped newlines in expressions are preserved

[coveralls]

Coverage remained the same at 100.0% when pulling 7b54847 on simplify_expression_parsing into a72c72d on master.

[asottile]

After this, the compiler + parser are under 1500 lines which is quite the improvement

$ wc -l Cheetah/legacy_*.py
  566 Cheetah/legacy_compiler.py
  783 Cheetah/legacy_parser.py
 1349 total