Test and document PC_MAX_CANON.

Closes issue #55.

I suspect since stdin is not a tty for travis,
that it is "failing", but try Trying maximum
of either/or PC_MAX_INPUT as well before I
go forcing icanon.
Merge branch 'doc-and-test-max-canon' of github.com:pexpect/pexpect into doc-and-test-max-canon