Add lehs.common

Add namespace (and file) lehs.common.  Only holds a single function
split-at-subseq.  Takes a sequence and returns a tuple of the contents
of the sequence up to and including the first occurrence of subsequence,
and a lazy sequence of the remaining elements.
fix namespace clash and refactor

rename get-method to method-keyword to fix method name clash.  Use new
function split-at-subseq in read-head (split-at-subseq is a generic
implementation of what read-head did before).
Update .gitignore

add some dot files to .gitignore