Use $MY_HOME in .emacs

To use .dotfiles with a shared user account,
it is convenient if $MY_HOME can be used as $HOME.
Use $MY_HOME generally in .bashrc to adapt diverse environments
Move sourcing .bash_aliases.local from .bashrc to .bash_aliases
Enable use of .bashrc.local

gitで共有すべきでない環境固有のbash設定を，
.bashrc.localに書けるようにした。
(.bashrc.localそのものはgit管理対象から明示的に除外)