Add happybase.filter with an escape() function

The new happybase.filter module will include helper routines to
construct Thrift filter string. For now, only an escape() routine has
been implemented. Includes test.
Add helpers for constructing Filter strings

Implement helpers for constructing filter strings. Added all built-in
filters and support for serialising integers, strings, and comparison
operators. Includes various tests.
Include happybase.filter in the test coverage report
Add happybase.filter module to API docs
Expand tutorial with section about advanced scanner filters
Update NEWS
Use byte strings in happybase.filter.escape()
Add support for booleans as filter arguments
Doc updates
Add filter string helpers for AND and OR

Added AND(...) and OR(...) callables, and implemented overloaded
operators for & and | for Filter instances.
Use filter with shorter name for the tests
Add filter string helpers for SKIP and WHILE
Add TODO item about comparators in filters
PEP8
Simplify test code
Simplify unary operator rendering
Add classes for all filter operators
Make _format_arg() a function
Drop parentheses around boolean operators
Support >2 operands for boolean operators