Add integration between TCD and NERDTree

Put g:TCDUsedInNERDTree in your .vimrc to let the TCD plugin handle
the "cd" command in NERDTree. This way, the current directory will only
be changed for the current tab.
Use TCD - NERDTree integration.
Change TCD to accept expressions.

:TCD getcwd() now works, whereas :TCD '%:p:h' no longer works without
quoting.
Ignore magic buffers for auto TCD plugin, cleanup

Plugins like NERD Tree and Bufstop open a fresh buffer, thus triggering
the Buf* autocommands. The autotcd plugin would see this and initialize
the TCD to the starting home directory.

Comment out some leftover code, add extra debug output by means of a
global variable (NERD Tree seems to be able to suppress output to
:messages).