Fix #89

Backspace after a failed history search made searching always fail
until starting a new interactive history search.
Add SelectAll function (Ctrl+A)

Fixes #84
Add function CopyOrCancelLine

Ctrl+C should copy selected text if any is selected.  Otherwise
it will cancel editing.

On the off chance somebody prefers the old behavior, CancelLine
is unchanged, but it is no longer bound.

Fixes #87
Copy all module files to build dir
Add "*.bak*" to .gitignore.
Replace slow DateTime.Now calls with Stopwatch (QueryPerformanceCounter)

It's an order of magnitude faster. We don't need all the fancy time info
given by DateTime.Now anyway; we just need an accurate timestamp.

There's also a correctness issue: when dealing with TimeSpans (what you
get when subtracting DateTimes), the Milliseconds property is not the
same as the TotalMilliseconds property.
Mark ReadKeyThreadProc thread as background to prevent hang on exit.

PowerShell.exe does not exhibit the hang because it hosts the CLR and does
some teardown differently, but pure-managed hosts will get stuck waiting
for this thread unless it is marked as a "background" thread.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	PSReadLine/PSReadLine.csproj