enable echo deactivation in pxssh

I've learned from #112 that you can deactivate echoing of the input via
constructer parameter. This patch enables pxssh to do the same.

In spawn there is an alternative to the constructor parameter, which is
`set_echo(False)`. Sadly that doesn't work, at least in my experiments with
localhost.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>
Merge pull request #114 from erikb85/master

enable echo deactivation in pxssh
Initial stab at asyncio integration
Refactor expect code
Expose async as parameter for expect
Fix EOF in async on Linux
support timeout in async expect
Expand tests for async expect
Check already read data before async reading more
Expose async parameter for expect_exact
Document asyncio integration
Stop testing on Python 2.6 and 3.2

We will drop support for these Python versions in Pexpect 4.0
Merge pull request #69 from takluyver/asyncio

asyncio integration for expect
Add Pexpect development team to copyright statement

Closes gh-117
Closes gh-118