improve load-file error message
add $PWD to the load-paths by default

this allows loading "project-local" namespaces and files easily.
don't fail when being symlinked to
try finding the binary in $PATH if it isn't in the current directory
don't use load-file for batch mode

it's confusing because it interacts with load-paths in strange ways.
(load-file doesn't simply load a file and evaluates it, but it looks
through load-paths and tries to locate it in there.)
print an error if file isn't present

doesn't work with inaccessible files, though.
renamed extensions to .pxi
Merge pull request #71 from heyLu/load-paths

Improve load path/batch mode
Merge pull request #73 from andrewchambers/pxi

renamed extensions to .pxi
Merge remote-tracking branch 'upstream/master' into functions