Match intent of Export#filename test to signature

The previous test's description did not match its use in the test;
it said the Method Under Test's implementation would use today's date
but instead used the value of the constructor's parameter.

Since the application never uses the second parameter in the constructor,
it looks like the parameter was only used for testability.

We can instead safely stub today's date using Timecop.

Also:

* Use shorter, Law of Demeter-friendly `Date.current`.
* Since the Method Under Test is `#filename`,
  invoke it during the Exercise phase,
  separated by blank lines.
Merge pull request #122 from croaky/dc-spec

Match intent of Export#filename test to signature
Strip replies and signatures from emails

Most of this stuff should get picked up by Griddler, but if any slips
though, let's take care of it with GitHub's secret sauce.
Merge pull request #125 from codecation/no-sigs-within-10-feet-please

Strip replies and signatures from emails
Place interesting entries in the prompt email

Entries from 1 year ago, 1 month ago, 1 week ago are more interesting
that random entries.
Merge pull request #124 from codecation/focussed-reflection

Place interesting entries in the prompt email
Move partial

We don't really have a canonical view for entries, since we render them
differently in search results and on the dashboard.

This commit moves the entries/_entry partial into searches/.
Remove README note about Heroku access

[Suspenders]' (the generator used to create the Trailmix Rails app)
`./bin/setup` script doesn't "fail" in a non-zero exit status sense.

[Suspenders]: https://github.com/thoughtbot/suspenders

The `heroku join` lines are protected by `if` statements:

```bash
if heroku join --app trailmix-staging &> /dev/null; then

if heroku join --app trailmix-production &> /dev/null; then
```

The failing branch of the conditionals used to print messages to `stdout`:

> Ask for access to the "trailmix-staging" Heroku app
> Ask for access to the "trailmix-production" Heroku app

This commit removes those messages to avoid a situation where
open-source contributors ask for access to the Heroku apps.

Don't add remotes for the Heroku applications if user doesn't have access.
Open source contributors don't have access and won't want/need the remotes.