Replace pos-tip with popup.el, more tests

This has been my dev branch over the last week, so I can split this up into smaller pull requests if you don't want everything.

### Replace pos-tip with popup
pos-tip is not compatible with terminals. It also draws to random locations on the screen for me on OS X. I've replaced it with popup, which works in terminals. It may also give us better control over text properties for prettifying the popups, though I haven't looked into that side of things.
Popup is the display backend used by autocomplete, so you may be taking an indirect dependency on that anyway.
The only problem I've noticed so far with popup is that it will wrap in strange ways if the terminal window is too small.

### Distinguish tooltip requests from showing type signatures
Minibuffer type signatures are now always displayed. C-c C-t will display the full tooltip, either in a popup or in a help buffer.

### Improved test coverage
I've introduced a suite of unit tests for fsharp-mode-completion. These stub out interaction with the background process, so it should make you less reliant on the integration tests. I've also unit tested the additions in this branch.

### Makefile changes
Added `all` task to download package dependencies and build ac-process.