Added gitignore

Templated from https://github.com/github/gitignore
Hypermail 'library' autotools changes

Modified `Makefile.in`s, `configure.in`s to:
1. ignore builtin `pcre` library
2. support normal `configure`&`make` build flow
Remove src/pcre library folder from the project
Documentation updated
Merge pull request #11 from rampage644/dev

Making `hypermail` library more autotools friendly