address all R CMD check notes

Leaving these unaddressed for what I think are good reasons:
- Won't fix `require('rstan')` call because `requireNamespace` isn't enough. `rstan::stan_model()` and/or `rstan::sampling()` can't find the rstan function `cpp_object_initializer()` unless the namespace is loaded. `requireNamespace` is somehow not doing this. An online discussion (without solution) is at https://stat.ethz.ch/pipermail/r-devel/2014-September/069803.html

``` r
* checking dependencies in R code ... NOTE
'library' or 'require' call to 'rstan' in package code.
Please use :: or requireNamespace() instead.
See section 'Suggested packages' in the 'Writing R Extensions' manual.
```
- Won't fix `:::` calls to `'devtools:::github_resolve_ref'` and `'devtools:::parse_git_repo'` because (1) I'd have to copy devtools code to get the functionality I want without the `:::` - I want to get the name of the most recent tag on a GitHub branch - and (2) these calls occur in an unimportant part of the package (`.onAttach`), are wrapped in a `tryCatch`, making it unlikely that changes to the unexported functions would damage anybody's workflow.

``` r
* checking dependencies in R code ... NOTE
Unexported objects imported by ':::' calls:
'devtools:::github_resolve_ref' 'devtools:::parse_git_repo'
See the note in ?`:::` about the use of this operator.
```
- Won't fix call to `attach()` in `revise()` because I really want the clean interface I get from using `attach()`, have tried and failed to achieve it using `eval` and `evalq` and `with`, and believe I am following the best practices outlined in `?attach`.

``` r
* checking R code for possible problems ... NOTE
Found the following calls to attach():
File 'streamMetabolizer/R/revise.R':
attach(specs, warn.conflicts = FALSE, name = "revisespecstemp")
See section 'Good practice' in '?attach'.
```