pass ... from list2df through to as.data.frame

e.g. for stringsAsFactors
fix collection of connection number
Add catmaid_get_connectors()
Make an environment on startup to store connections
Cache connection details on login
name cached connections with cookie

* since there could be multiple connections for the same server
  (different users?)
new private function to fetch cached connection

* needs to match whatever fields we have filled in
test fetching cached connections (some failures)

* turns out that catmaid_connection is using default values for e.g. username/pwd
  even if these clearly don't apply because they relate to another server
fix: catmaid_connection should only use consistent defaults

* closes #2
Use cached connection unless Force=TRUE
tests no longer need to cache connection in options

* was using a non-standard option as a hack to store open connection info
roxygenise inc more help for cached catmaid_login
make a single environment to store package state

* also add function to cache connection in this new env
* reason to do this is to know who was last connection
Add catmaid_last_connection()

* retrieves the last connection to be cached
catmaid_login uses last connection absent info

* in the absence of any specific connection information catmaid_login will
  use the last cached connection information
Update help in catmaid_login re cached connections
Proper package docs
create/export connections to/from environment vars
fix typo
use catmaid_connection_getenv() in tests