Slightly robustified installer that passes adb flags; added update, delete scripts.

I came across your awesome Chromium Test Shell installer this morning and made a few modifications for myself. Here you go, if you're interested!

Most notably, the script tracks requirements and aborts (`exit 1`) if they're unmet, like `curl` failing to download the package. This should make it slightly more reliable in automated, unattended or triggered workflows.

I also added a simple updater and deleter (the latter of which is totally unnecessary but what the heck…)

I've tested it out in the emulator and my physical Nexus 7: both seem to work fine.

:warning: I'm not entirely confident on the use of `adb shell pm uninstall -k org.chromium.chrome.testshell` on update (line 18) and delete (line 14), but `adb` complained when I tried `adb uninstall -k` by itself. Please do fix that if I've gotten it wrong.