Convert HttpParser to dynamic reference counted class

This creates a dedicated HttpParser object for each new request in
preparation for incremental parsing of the client connection buffer.

NP: incremental parse is not yet done so when requests require multiple
  calls of the parse routine it will for now generate a new parser state
  object and discard the previous partial parse details.
Make Http parser incremental

This makes the parser track the start of the un-parsed buffer region and
resume parsing at that point instead of reset to the beginning.

- add state to mark completion of the first line (request-line)
- add unit test for incremental parse states including BWS
- make the state tracking member of HttpParser private
Add parser/libsquid-parser.la

Also, basic class CharacterSet definition
Update CharacterSet.h
Merged from trunk
interim: better adhere to CharacterSet API, first stubs of Tokenizer
Fist stub of tokenizer unit tests
implemented untested Tokenizer::find_first_not_in and Tokenizer::prefix
Fix tokenizer unit tests scaffolding
Reimplemented prefix, implemented prefix matching unit test
Fix infinite-loop in CharacterSet::operator+=()

The uint8_t type value cannot represent the value 256, which is why we
use it as the loop terminator and an iterator of a size larger than
8-bit with a down-cast to prevent the vector accidentally being grown by
an overflow.
Merged from trunk
Broken: try and figure out CharacterSet::operator +=; more Tokenizer::prefix unit tests
Fixed initialization problem in testCharacterSet
Removed debug code in testTokenizer
Completed testTokenizerPrefix
Implemented more Tokenizer methods and unit tests
Implemented Tokenizer::token and its unit test
Merged from trunk
Merged from trunk
Merged from trunk
Documentation tweak